Solid phase extraction is a powerful technology for purifying and concentrating analytes, including biomolecules. For example, it is one of the primary tools used for preparing protein samples prior to analysis by any of a variety of analytical techniques, including mass spectrometry, surface plasmon resonance, nuclear magnetic resonance, x-ray crystallography, and the like. With these techniques, typically only a small volume of sample is required. However, it is often critical that interfering contaminants be removed from the sample and that the analyte of interest is present at some minimum concentration. Thus, sample preparation methods are needed the permit the purification and concentration of small volume samples with minimal sample loss.